Serum total cholinesterase activity, dibucaine inhibition of the enzyme activity, and red cell acetylcholinesterase activity were also measured at intervals after the ingestion, with the results indicated below.

.

Time AfterIngestion (Hrs.)

Value

ReferenceRange

Units

CHOLINESTERASE

1434

83155

1600-5500

IU/L

DIBUCAINEInhibition

1434

1753

75-83

% Enzyme Inhibition

ACETYL-CHOLINESTERASE

88

127

100-288

U/Min/GM HB

Chlorpyrifos Mass Spectra

Organophosphate Insecticide Poisoning

Both cholinesterase and acetylcholinesterase can be irreversibly inhibited by binding to organophosphate insecticides, such as diisopropylfluorophosphate (DFP), ParathionTM, SavinTM, and tri-o-cresyl phosphate. The inhibition occurs when the organophosphate compound, whose structure resembles the transition state for acetyl-ester hydrolysis, binds strongly to the enzymes catalytic site. The inhibition is observed in the laboratory as a decrease in serum cholinesterase activity following poisoning. Acetylcholinesterase activity, as measured in red cells, is also decreased, and may be a better marker for insecticide poisoning since some individuals exposed to the toxins may have normal or near normal levels of total plasma cholinesterase.

A major consequence of the inhibition of acetylcholinesterase is that increased concentrations of acetylcholine are present at synaptic junctions using the transmitter. At sites at which this occurs, the membrane of the post-synaptic cell repeatedly depolarizes, producing action potentials in post-synaptic neurons and muscle contraction in post-synaptic skeletal and smooth muscle. In the skeletal and smooth muscle, the intracellular calcium released during each depolarization eventually builds up to the point that the muscle can no longer relax. This effect in skeletal muscle is observed clinically as muscle fasciculations followed by paralysis.

The effects on the smooth muscle of the gastro-intestinal and respiratory tracts are also prominent, due to a combination of increased acetylcholine at the muscarinic receptor sites in the autonomic ganglia and at the motor endplates. Clinically, the patient exhibits excessive salivation, nausea, vomiting, diarrhea, abdominal cramping, and asthmatic symptoms related to a combination of bronchoconstriction and increased respiratory secretions. Similar effects at other sites produce excessive sweating and pupillary constriction with ocular pain. Effects on the cardiovascular system can produce potentially dangerous bradycardia, arrhythmias, and pulmonary edema. Overstimulation of muscarinic sites in the central nervous system can cause confusion, ataxia, slurred speech, Cheyne-Stokes respiration, convulsions, coma, and central respiratory paralysis.

Death after a single acute exposure to organophosphate insecticides may occur at between 5 minutes and 24 hours. The usual cause of death is respiratory failure, often accompanied by cardiac manifestations.

Organophosphate insecticide poisoning is usually amenable to therapy. Atropine acts on the muscarinic acetylcholine receptors to decrease their sensitivity to acetylcholine. Administration of atropine can consequently ameliorate many of the effects of the inhibitors of cholinesterase, including the effects on the respiratory, gastro-intestinal, and cardiac functions. Unfortunately, atropine has no action at the nicotinic receptors of the skeletal muscle, so the paralysis cannot be reversed by atropine. Also, atropine enters the central nervous system poorly, and blocking of central effects is therefore difficult.

Pralidoxime is also used in the therapy of organophosphate poisoning. This drug acts to break the phosphate bond between the insecticide and the acetylcholinesterase, thereby "reactivating" the cholinesterases at a more rapid rate than would normally occur. The development of this drug represents one of the many successes of modern medicine: Wilson and Ginsberg predicted that a site-directed nucleophil might break the bond between the insecticide and the enzyme, and they then designed, synthesized, and tested pralidoxime in 1955.